During the years, electric machine designs evolved from modulated pole machines, claw pole machines, Lundell machines and transverse flux machines (TFM) have become more and more interesting. Electric machines using the principles of these machines were disclosed as early as about 1910 by Alexandersson and Fessenden. One of the most important reasons for the increasing interest is that the design enables a very high torque output in relation to, for instance, induction machines, switched reluctance machines and even permanent magnet brushless machines. Further, such machines are advantageous in that the coil is often easy to manufacture. However, one of the drawbacks of the design is that they are typically relatively expensive to manufacture and that they experience a high leakage flux which causes a low power factor and a need for more magnet material. The low power factor requires an up-sized power electronic circuit (or power supply when the machine is used synchronously) that also increases the volume, weight and cost of the total drive.
The modulated pole electric machine stator is basically characterised by the use of a central single winding that will magnetically feed multiple teeth formed by the soft magnetic core structure. The soft magnetic core is then formed around the winding, while for other common electrical machine structures the winding is formed around the tooth core section. Examples of the modulated pole machine topology are sometimes recognised as e.g. Claw-pole-, Crow-feet-, Lundell- or TFM-machines. The modulated pole machine with buried magnets is further characterised by an active rotor structure including a plurality of permanent magnets being separated by pole sections. The active rotor structure is built up from an even number of segments, whereas half the numbers of segments are made of soft magnetic material and the other half number of segments are made from permanent magnet material. The permanent magnets are arranged so that the magnetization direction of the permanent magnets is substantially circumferential, i.e. the north and south pole, respectively, is pointing in a substantially circumferential direction.
The state of the art method is to produce the rather large number of rotor segments, typically 10-50 individual pieces, as individual components. The assembly of these segments is further complicated by the opposing polarisation direction of the permanent magnet segments that will tend to repell the pole sections from each other during the assembly. The magnetic functionality of the soft magnetic pole sections in the desired machine structure is fully three-dimensional and it is required that the soft magnetic pole section is able to efficiently carry magnetic flux with high magnetic permeability in all three space directions. A traditional design using laminated steel sheets will not show the required high permeability in the direction perpendicular to the plane of the steel sheets and it is here beneficial to use a soft magnetic structure and material which shows a higher magnetic flux isotropy than a state of the art laminated steel sheet structure.